1. Field of the Invention
This invention relates generally to the structures of a fast recovery rectifier. This invention discloses the method of reducing the reverse recovery time from a conventional fast recovery rectifier by either introducing the Schottky structure at the front and/or the back of the substrate or introducing the junction termination layer to reduce the total space charge (or built-in potential) of the p-n junction. Furthermore, this invention also provides the low cost manufacturing of a very fast recovery rectifier.
2. Description of Related Art
FIG. 1 shows the cross section of a conventional fast recovery rectifier. The doped substrate 100 is made by either n type or p type material. The doped substrate 100 used is a wafer with sawed or unpolished rough surface treatment for cost savings. The polish of the wafer surface is not required in this case. Doping of wafer can be done by either film method or by standard deep diffusion method. The p-n junction 103 can be formed on the doped substrate 100, and the depth of the p-n junction 103 is usually from less than 10 microns to over 20 microns so that the depth of the p-n junction 103 is deeper than the surface damaged region of the rough surface. Then the lifetime killer such as Pt, Au, etc. is added to the wafer with thermal treatment. The reverse recovery time is depending on doping concentration, the thermal treatment temperature, and time of the lifetime killer species. The other method for the lifetime reduction can also be done by irradiation of electrons or other species. After the irradiation treatment, the substrate is being processed by annealing.
The diffusion to the doped substrate 100 is generally using opposite polarity species at the p-n junction 103. The depth of this p-n junction 103 should be deeper than the surface damage region for the reduction of the leakage current. The diffusion to the doped substrate 100 is generally using the same polarity as at the backside layer 101. The passivation layer 104 can be formed after the formation of the p-n junction 103 as well as the life time killer process. The irradiation of the electrons and or other species can be done after the process of the wafer or even in the packaged parts. The passivation layer 104 can be done by conventional glass passivation process or multiple CVD process. The top metal layer 105 is deposited, evaporated, sputtered, or plated along with the top of the p-n junction 103. The bottom layer 105B metallization is to be done by similar metallization process or by nickel plating. After the completion of the process, the wafer is diced into chips for assembly.
FIG. 2 shows the cross section of an epitaxial based fast recovery rectifier in prior art. The epitaxial layer 102 has the same polarity and is grown on the heavily doped substrate 100. The doping concentration and the thickness of the epitaxial layer 102 are determined by the breakdown voltage. The epitaxial layer 102 can be made by single or multiple layers. The p-n junction 103 anode diffusion can be done by either ion implantation or diffusion method of the opposite polarity to the epitaxial layer 102. After the formation of the p-n junction 103 anode diffusion, the life time killer such as Pt, Au or other species with proper thermal treatment can be added to the wafer. After the p-n junction diffusion, the deep etched structure is to be done by wet etch prior to the passivation process. The passivation layer 104 is done either by the conventional glass passivation or multiple CVD layers method. The top metal layer 105 is then opened for the metallization. The top metal layer 105 can be done by the contact metallization using either Ti—TiN—Al, TiNiAg or Nickel plating for either wire bond or soldering. The backside layer 101 can be done by the implantation of similar polarity to the silicon doped substrate 100 or omitted if the doped substrate 100 is heavily doped. The bottom layer 105B metallization is done either by Ti—Ni—Ag or Cr—Au or by Ni plating. After the completion of the process, the wafer is then ready for the dicing and assembly.
U.S. Pat. No. 6,261,874, Francis and Ng disclosed the fast recover diode structure with both Beam Radiation defects and He implanted defects to reduce the reverse recovery time. With this structure, the soft recovery time can result. U.S. Pat. No. 6,486,524 Ahmed disclosed the complicated structure using p-n junction as well as the Al or Pd Schottky for the fast reverse recovery time. U.S. Pat. No. 6,603,153, Francis and Ng disclosed the fast recovery rectifier structure that is similar to U.S. Pat. No. 6,261,875. U.S. Pat. No. 6,699,755 Bol disclosed a fast recovery diode structure similar to U.S. Pat. No. 6,486,524. U.S. Pat. No. 6,870,199 Yoshikawa et al disclosed the multiple life time control region for the improvement of di/dt caused breakdown. U.S. Pat. No. 6,927,141 Andoh et al disclosed the termination structure by using equal metal ring.